Several publications are referred to in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein through appropriate citation.
Electrochemical capacitors (ECs) are of growing interest in the field of energy storage devices due to the combination of high specific energy and specific power, in comparison with batteries and conventional capacitors. In batteries, the specific energy (usually given in units of watt-hour/kg) is high, but the specific power (usually given in units of watts/kg) is low, whereas in conventional capacitors, the specific power is high, but the specific energy is low. ECs are complementary to the pulse batteries and offer applications in high power applications, hybrid-power systems for electrical vehicles, telecommunication devices, memory backup, stand-by power systems, on-board power supply, and energy storage at the extreme conditions (e.g., in deep mines and military applications). The most important feature is that they have excellent reversibility and a much longer life cycle (˜20 years) than batteries.
In particular, ECs, sometimes called ultracapacitors, or supercapacitors, are of interest in hybrid electric vehicles, where they can supplement a battery to provide bursts of power needed for rapid acceleration. The latter is the biggest technical hurdle to making battery-powered cars commercially viable. A battery would still be used for cruising, but capacitors would kick in whenever the car needs to accelerate for merging, hill climbing, passing, and emergency maneuvers. This is because ECs have high power densities, i.e., they release energy much more quickly than batteries. In electric vehicle applications, large ECs can “load-level” the power demanded of the battery system, and thereby increase battery life and extend power density to a range needed in a transport vehicle. ECs can play a similar supplementary role in automobiles projected for the future, wherein a combination of fuel cells and ECs would form a part of the power train that includes an internal combustion engine. Such a hybrid power train combines greater fuel efficiency with reduced consumption of fossil fuels. To be cost- and weight-effective compared to additional battery capacity (as an alternative), ECs must combine adequate specific energy and specific power with long cycle life, and must meet the target of low cost as well.
Electrochemical capacitors are gaining acceptance in the electronics industry as system designers become familiar with their attributes and benefits. Compared to conventional capacitors, ECs have very high capacitance values, a relatively limited frequency response, and a relatively high equivalent series resistance (ESR), which is determined by electrode-electrolyte interface, electrode thickness, adhesion properties of the electrode, and the cross sectional area of the electrode. ECs were originally developed to provide large bursts of driving energy for lasers in satellites. In complementary metal oxide semiconductor (CMOS) memory backup applications, for instance, a one-Farad EC having a volume of only one-half cubic inch can replace nickel-cadmium or lithium batteries and provide backup power for months.
Conventional capacitors store energy in the electric field between two oppositely charged, conducting parallel plates, which are separated by an insulator. The amount of energy a capacitor can store increases as the area of conducting plates increases, the distance between the plates decreases, and the dielectric constant (the ability to store charge between the plates) of the insulating material increases.
Conventional electrochemical energy storage is achieved in a galvanic cell or a battery of such cells. The energy corresponds to the charge associated with chemical redox changes that can occur in the battery on discharge, multiplied by the voltage difference between the electrodes of the cell. The discharge process involves a net chemical reaction in the cell associated with passage of a certain number of electrons per formula unit, or faradays per mole of reactants.
ECs do not approach batteries in the energy density. For a given applied voltage, capacitatively storage energy associated with a given charge is about half that storable in a corresponding battery system for passage of the same charge. This difference is due to the fact that, in an ideal battery reaction, involving two-phase systems, charge can be accumulated at constant potential while, for a capacitor, charge must be passed into the capacitor where voltage and charge are being continuously built up. This is why energy storage by a capacitor is about half that for the same charge and voltage in battery energy storage under otherwise identical and ideal conditions. This makes ECs energy storage mechanism reversible, as change in energy is continuous, making the work done reversible (from a thermodynamic point of view), and there is no involvement of phase separation.
Despite their energy density being lower than that of batteries, ECs are extremely attractive power sources. Compared to batteries, they require no maintenance, offer much higher cycle-life (due to the reversibility referred to above), require a very simple charging circuit, experience no “memory effect”, and are generally much safer. Physical rather than chemical energy storage is the key reason for their, safe operation and extraordinarily high cycle-life.
Interest in automotive starting, lighting and ignition (SLI) applications, as well as in electric vehicle (EV) load-leveling, has stimulated product development activities for such high-power devices. The goal is to develop devices that can be efficiently charged and then discharged within the short duration specified for these high-power applications.
Severe demands are placed on the energy storage system used in an EV. The system must store sufficient energy to provide an acceptable driving range. It must have adequate power to provide acceptable driving performance, notably acceleration rate. In addition, the system must be durable to give years of reliable operation. And finally, the system must be affordable. These four requirements are often in conflict for candidate energy storage technologies. This situation creates significant challenges to developers of EV energy storage systems.
A capacitor offers significant advantages to the EV energy storage system. But, to be useful, it must store about 400 Wh of energy, be able to deliver about 40 kW of power for a short duration, provide high cycle-life (≧100,000 cycles), and meet specified volume, weight, and cost constraints.
The present invention is the result of an effort to address these requirements, and concerns developments in the direction of fulfilling some of them.
The energy stored in a charged capacitor can be continuously increased in proportion to the increase of the voltage, limited only by electrical breakdown of the dielectric. The maximum available stored energy, for a given chemical species, is determined by the quantity of electrochemically active materials, their standard electrode potentials, and their equivalent weights, while the maximum power is limited by the reversibility of the electrochemical changes that take place over discharge, together with the electrical resistivity of the materials and external circuitry.
Electrochemical capacitors are classified on the basis of different mechanisms, namely the double layer mechanism and the pseudo-capacitance mechanism. The double layer capacitance mechanism arises from the separation of the charge at the interface between a solid electrode and an electrolyte, and chemisorption and desorption of the electrolyte takes place over an appropriate potential range. The charge storage process is non-Faradaic, i.e., ideally, no electron transfer takes place across the electrode interface [1-4]. The electric double-layer capacitance of a metal electrode is in the order of a few tens of μF/cm2, and to make use of the electric double-layer capacitors in applications requiring large capacitance, one must consider high surface area electrode materials. Activated carbon electrode-based capacitors are based on double layer capacitance mechanism, where high capacitance depends on large surface area, stated usually in m2 of surface area of the electrode per gram of the electrode material [5-9]. Although the energy storage capability of the double layer was recognized more than 100 years ago, it took the development of low-current-draw volatile computer memories to create a market for ECs.
The Helmholtz double layer plays a key role in the working of the EC. It forms near the electrode/electrolyte interface. When an electrode has excess charge, charge separation takes place in the electrolyte with the formation of a double layer as shown in FIG. 1. This idea is similar to that of an electrolytic capacitor, where charge separation takes place only by pure electrostatic force. The outer layer is formed by the first layer of non-specifically adsorbed ions. These ions are completely surrounded by the solvation shell. The thickness of the outer layer is the distance from the centre of non-specifically adsorbed ion and the electrode. These ions are held in place by purely electrostatic forces and give electrostatic contribution to the capacitance. The inner layer is formed by specifically adsorbed ions, which are ions with a weakly bounded solvation shell. These ions may lose some part of the solvation shell to form a chemical bond with the electrode surface. The chemical interaction between specifically adsorbed ions and the electrode surface causes more charge to be accumulated at the surface than required by electrostatics, giving rise to a very large capacitance [1,10].
The high volumetric capacitance density of an EC (10 to 100 times greater than conventional capacitors) derives from using porous electrodes to create a large effective “plate area” and from storing energy in the diffuse double layer. This double layer, created naturally at an electrode-electrolyte interface when voltage is imposed, has a thickness of only about 1 mm, thus forming an extremely small effective “plate separation”, (and thus high capacitance).
The performance characteristics of electrochemical capacitors are fundamentally determined by the structural and electrochemical properties of electrodes. Various materials, including doped conducting polymers, metal oxides, metal nitrides, and carbon in various forms, have been studied for use as electrode materials.
Several methods are known in the art for increasing the amount of energy stored in an electrochemical capacitor. One such method is to increase the surface area of the active electrode. High surface area electrodes result in an increase in capacitance and thus increased stored energy. Another approach to increasing stored energy involves using different types of material for fabricating the capacitor electrodes. Carbon electrodes are used in most commercial capacitors, while precious metal oxide electrodes are used in the capacitors known as pseudocapacitors.
The pseudocapacitance mechanism arises indirectly from fast reversible interfacial pseudo-faradaic redox reactions in which chemisorption of ions or molecules takes place with partial charge transfer over an appropriate potential range i.e., the electrode material partially gets oxidized or reduced [1,11,12]. Pseudocapacitance can add significantly to enhancing the capacitance—and hence energy density—of the supercapacitor device.
An electrochemical capacitor involving pseudo-faradaic reactions will have a cyclic voltammogram different from that of a pure double-layer capacitor, the pseudocapacitance revealing a Faradaic signature. Doublelayer capacitors are commonly of the order of a few tens of μF cm−2, while pseudocapacitors associated with EC systems are commonly of the order of hundreds of μF cm−2 [1,11,12].
There are generally two kinds of pseudocapacitor electrode materials: conducting metal oxides, (i.e., RuOx, IrOx, and CrOx, x˜2.0) and redox conductive polymers (i.e., polyaniline, polypyrrole, and polythiophene). Pseudocapacitors fabricated to date with these electrode materials generally suffer from high material cost and low cell voltage. Supercapacitors with metal oxide electrodes commercially available today are expensive, as many of the preferred metals, such as Ru and Ir, are expensive. Supercapacitors with redox polymer electrodes generally have a relatively high energy storage capacity and low cost. However, these conductive polymers have a narrow working voltage in proton-conducting electrolytes and suffer from limited cycle life (degradation).
Supercapacitors with electrodes comprising hydrated ruthenium oxide, RuO2.nH2O, for have been shown to provide high capacitance because of the pseudocapacitance mechanism [13]. The amorphous nature of the hydrated oxide and the interaction between the proton of the hydroxyl group in the electrode and the electrolyte, together lead to fast proton diffusion rates which is responsible for high capacitance in hydrated ruthenium oxide. But high cost and limited cycle life are continuing impediments to the commercial use of such materials. Therefore, efforts have been made recently to prepare electrode materials containing a composite of metal oxides to reduce cost, to provide firm support for conduction, as well as to enhance the surface area.
Because of the redox reactions involved in the pseudocapacitance mechanism, the cyclic voltammogram for the pseudocapacitance mechanism-based capacitors may not be perfectly rectangular in shape. So, the durability of the capacitor based on pseudocapacitance mechanism will be limited in comparison to the capacitor based on the pure double-layer mechanism. Nevertheless, the working life of capacitors will be longer than that of batteries, wherein a distinguishable phase transition is observed in the cyclic voltammogram. On the other hand, using known or proposed electrode materials, the specific goals of obtaining high power output suitable for electric vehicle (EV) applications cannot be met by a pure double-layer capacitor. Carbon-based electrodes give slightly better capacitance in sulfuric acid, but oxidation of carbon and corrosion of the current collectors can be a problem, especially at higher voltages. This has made it necessary so far to employ expensive noble metals as the supporting conducting materials [14,15].
Because of its superior electrochemical properties, manganese dioxide (MnO2) is one of the promising, alternative metal oxide electrode materials. Its natural abundance, low cost and its environmental compatibility make it a very desirable choice in any energy storage device [6,16-21]. Various groups have reported experimentally deliverable capacitance of the order of 700 F/g [22]. However, the electrical conductivity of MnO2 is not as high as that of ruthenium oxide, which has the advantage of bulk conductivity throughout the electrode. Whereas the entire bulk is involved in hydrous ruthenium oxide in the pseudocapacitance mechanism, only the surface is involved in the case of manganese oxide [23]. One way to increase conductivity is to add a conducting additive such as carbon black [24-27].
The aforesaid points to the desirability of having a carbonaceous composite material, preferably a metal oxide/carbon composite, as the electrode material for supercapacitor. Furthermore, a composite material containing oxide nanoparticles in a carbonaceous matrix might help overcome the disadvantage noted above, viz., only the surface layers of the oxide (MnO2) is involved in the pseudo capacitance mechanism.